1. Field of the Invention
The present invention relates to a receiving level measuring circuit for use in a receiver of a mobile communication system, and more particularly to a receiving level measuring circuit capable of improving measuring accuracy of a receiving level and of meeting the specifications for the use of a receiver having a plurality of branches.
2. Description of the Related Art
In a mobile communication system, various multiple access methods have been invented to utilize resources of limited frequencies or the like, and a CDMA (direct sequence-code division multiple access) method is receiving attention.
In the CDMA method, particularly in a DS-CDMA (direct sequence code division multiple access) method in which individual spread codes are allocated to channels to be used for communications so as to be multiplexed and a pilot symbol is inserted into a transmission symbol before transmission while an amplitude phase variation is extracted from a despread signal of the pilot symbol and used to correct the received symbol for demodulation at a receiving end, it is known that there is a need for measuring a level of a received wave in a receiver in order to perform a closed-loop control type transmission power control that is characteristic of the DS-CDMA method.
A conventional general receiving level measurement for a level measurement of a received wave comprises detecting an RSSI (a received signal strength indicator) of a received-signal in a radio frequency band that has already been input to a receiver and calculating a dB value of the RSSI.
First, a sample structure of a receiving level measuring circuit in the conventional CDMA receiver will be described with reference to FIG. 9. Referring to FIG. 9, there is shown a block diagram illustrating a sample structure of the conventional receiving level measuring circuit.
As shown in FIG. 9, the conventional receiving level measuring circuit comprises an RSSI detection unit 1, an analog-to-digital conversion unit (shown as an A-D unit) 2, an RSSI averaging unit 3, and a voltage/dB conversion unit 4.
The following describes components of the conventional receiving level measuring circuit.
The RSSI detection unit 1 is used to detect a received signal strength indicator (RSSI) of a received signal in a radio frequency band and to output the voltage. Note that this unit is implemented in a commercially available RSSI detection IC.
The analog-to-digital conversion unit 2 is used to convert an RSSI analog value whose voltage has been output to a digital value.
The RSSI averaging unit 3 is used to average the detected RSSI.
The voltage/dB conversion unit 4 is used to convert the averaged RSSI voltage to a dB vale. Note that the function of this unit is implemented by previously generating a conversion table of the RSSI voltage versus the RSSI dB value and referencing it.
The following describes an operation of the conventional level measuring circuit with reference to FIG. 9.
In the conventional level measuring circuit, a received signal in a radio frequency band input to a receiver is input to the RSSI detection unit 1, an RSSI of the received signal is detected and output as an analog voltage value, the analog value is converted to a digital value in the analog-to-digital conversion unit 2, the value is submitted to a given averaging process in the RSSI averaging unit 3 and then converted to a dB value in the voltage/dB conversion unit 4, and a result of measuring the receiving level of the signal input to the receiver is output as an RSSI dB value.
This enables a measurement of a quality of receiving the signal input to the receiver.
The above conventional receiving level measuring circuit, however, has a problem that the signals input to the receiver are not always at desirable wave levels since a plurality of transmitters and receivers use an identical radio frequency band for communications in the DS-CDMA method, by which they are measured simply as receiving levels despite the fact that interference wave levels are included.
In addition, the RSSI measurement in the radio frequency band has a problem that it is impossible to measure levels lower than or equal to an in-band noise of a radio unit of the receiver in a theoretical sense, thereby disabling accurate measurement of receiving levels at all levels.
To solve these problems, there has been offered a technology titled xe2x80x9cReceiving level measuring method and receiving level measuring circuitxe2x80x9d (Applicant: Hitachi Kokusai Electric Inc., Inventor: Takahito Ishii) in Japanese Unexamined Patent Publication (Kokai) No. 285209 of 2001 laid open to public inspection on Oct. 12, 2001.
This conventional technology is a receiving level measuring method and a receiving level measuring circuit for detecting a field strength of a received signal, detecting a desirable wave and an interference wave for each spread code from a baseband signal through a quadrature detection, synthesizing a plurality of desirable wave components with each other to convert them into electricity, correcting the synthesized desirable wave level by adding a reception field strength detected in the radio unit if the reception field strength is more than the in-band noise while using the desirable wave level directly if the reception field strength is lower than or equal to the in-band noise, and considering it to be a result of the receiving level measurement.
The conventional technology in the above, the receiving level measuring-circuit in Japanese Unexamined Patent Publication No. 285209 of 2001 has a basic configuration enabling a measurement of a desirable wave receiving level up to a level lower than or equal to the in-band noise of the receiver radio unit since the measuring circuit separates and measures the desirable level and interference wave level and uses the desirable wave power level as a desirable wave receiving level if the reception field strength is a specific value or below (in-band noise or below) or corrects the desirable wave power level by adding the field strength level to the desirable wave power level and considers the corrected value to be a desirable wave receiving level if the reception field strength is more than the specific value (more than the in-band noise).
It is an object of the present invention to provide a receiving level measuring circuit capable of improving measuring accuracy of a receiving level by correcting phase rotation caused by a frequency error between oscillators included in a transmitter and a receiver and by inhibiting an inappropriate correction in situations that no desirable wave exists.
It is another object of the present invention to provide a receiving level measuring circuit adapting to a receiver configuration having a plurality of antennas (branches).
In accordance with one aspect of the present invention, there is provided a receiving level measuring circuit, comprising: received signal strength indicator (RSSI) detection means for detecting a field strength of a received signal, quadrature detection means for performing a quadrature detection of the received signal, desirable/interference wave detection means for detecting a plurality of desirable wave components, a plurality of interference wave components, and a plurality of frequency error vectors from the received signal submitted to the quadrature detection, frequency error correction-value acquisition means for previously storing a correction value corresponding to the frequency error and then adding and averaging the plurality of frequency error vectors detected by the desirable/interference wave detection means so as to get the correction value corresponding to the frequency error submitted to a unit conversion, frequency error correction means for adding the plurality of desirable wave components detected by the desirable/interference wave detection means and for correcting the desirable wave components using the correction value, desirable wave. component electricity conversion means for converting the corrected desirable wave components into electricity to get a desirable wave power level, and desirable wave level correction means for outputting the desirable wave power level as a desirable wave receiving level if the detected field strength is lower than or equal to a predetermined specific value or for correcting the desirable wave power level by adding the detected field strength level thereto and then outputting the corrected level as a desirable wave receiving level if the detected field strength is more than the specific value, thereby improving measuring accuracy of the desirable wave receiving level by correcting phase rotation caused by a frequency error between oscillators included in a transmitter and a receiver when the measurement is made by separating the desirable wave level from the interference wave level and if the reception field strength is the specific value or below (the in-band noise or below) the desirable wave power level is considered to be a desirable wave receiving level or if the reception field strength is more than the specific value (more than the in-band noise) the desirable wave power level is corrected by adding the field strength level to it to consider the corrected level to be a desirable wave receiving level.
In the above receiving level measuring circuit according to the present invention, it further comprises interference wave component averaging means for averaging a plurality of interference wave components detected by the desirable/interference wave detection means and for outputting an interference wave receiving level and signal-to-interference ratio (SIR) measuring means for getting a signal-to-interference ratio (SIR) based on the interference wave receiving level from the interference wave component averaging means and on the desirable wave power level, wherein, in a case that the detected field strength exceeds the specific value in the correction with reference to the specific value in the desirable wave level correction means, the desirable wave level correction means makes a correction of adding the detected field strength level to the desirable wave power level if the SIR is more than zero, but does not make the correction of adding the detected field strength level to the desirable wave power level if the SIR is zero or less, thereby improving measuring accuracy of the desirable wave components by preventing inappropriate correction when no desirable wave exists.
In accordance with another aspect of the present invention, there is provided a receiving level measuring circuit for measuring a receiving level of a received signal received at a plurality of antennas, comprising for each antenna: received signal strength indicator (RSSI) detection means for detecting a field strength of a received signal, desirable/interference wave component power output means for converting the received signal to a baseband signal, detecting a desirable wave and an interference wave for separation, and outputting a desirable wave power level and an interference wave power level, signal-to-interference ratio (SIR) measuring means for getting a signal-to-interference ratio (SIR) from the desirable wave power level and the interference wave power level, desirable wave level correction means for outputting the desirable wave power level as a desirable wave receiving level if the detected field strength is lower than or equal to a predetermined specific value or for making a correction of adding the detected field strength level to the desirable wave power level and a correction using a spreading ratio and a fixed correction value and outputting the corrected level as a desirable wave receiving level, and interference wave level correction means for correcting the interference wave power level using a spreading ratio and a fixed correction value and outputting the corrected value as an interference wave receiving level, and having desirable wave antenna synthesizing means for synthesizing desirable wave receiving levels output from the desirable wave level correction means for each antenna and outputting the synthesized desirable wave receiving level, interference wave antenna synthesizing means for synthesizing the interference wave receiving levels output from the interference wave level correction means for each antenna with weighting by using the SIRs in the corresponding antennas and outputting the synthesized interference wave receiving level, and synthesized SIR measuring means for getting a synthesized SIR from the synthesized desirable wave receiving level and the synthesized interference wave receiving level, thereby enabling a measurement of a receiving level up to a level lower than or equal to an in-band noise in a receiver radio unit so as to adapt to a receiver configuration having a plurality of antennas (branches).
In the above receiving level measuring circuit according to the present invention, the desirable/interference wave component power output means comprises quadrature detection means for performing a quadrature detection of the received signal and converting it to a baseband signal, desirable/interference wave detection means for detecting a plurality of desirable wave components and a plurality of interference wave components from the baseband received signal submitted to the quadrature detection, desirable wave component electricity conversion means for adding the detected plurality of desirable wave components, converting them into electricity, and outputting a desirable wave power level, and interference wave component averaging means for averaging the detected plurality of interference wave components and outputting an interference wave power level, thereby enabling a measurement of a receiving level up to a level lower than or equal to the in-band noise in the receiver radio unit so as to adapt to the receiver configuration having a plurality of antennas (branches) and getting very accurate values on both of the desirable wave and the interference wave.
Furthermore, in the above receiving level measuring circuit according to the present invention, the desirable/interference wave component power output means comprises quadrature detection means for performing a quadrature detection of the received signal and converting it to a baseband signal, desirable/interference wave detection means for detecting a plurality of desirable wave components, a plurality of interference wave components, and a plurality of frequency error vectors from the baseband received signal submitted to the quadrature detection, frequency error correction-value acquisition means for previously storing a correction value corresponding to the frequency error and then adding and averaging the plurality of frequency error vectors detected by the desirable/interference wave detection means so as to get the correction value corresponding to the frequency error submitted to a unit conversion, frequency error correction means for adding the detected plurality of desirable wave components and correcting the desirable wave components using a correction value, desirable wave component electricity conversion means for converting the corrected desirable wave components into electricity to obtain a desirable wave power level, and interference wave component averaging means for averaging the detected plurality of interference wave components and outputting an interference wave power level, thereby enabling a measurement of a receiving level up to a level lower than or equal to the in-band noise in the receiver radio unit so as to adapt to the receiver configuration having a plurality of antennas (branches), getting very accurate values on both of the desirable wave and the interference wave, and further improving measuring accuracy of the desirable wave receiving level by correcting phase rotation caused by a frequency error between oscillators included in a transmitter and a receiver.